1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine having a first fuel injection mechanism (an in-cylinder injector) for injecting a fuel into a cylinder and a second fuel injection mechanism (an intake manifold injector) for injecting a fuel into an intake manifold or an intake port, and relates particularly to a technique for determining a timing of ignition with a fuel injection ratio between the first and second fuel injection mechanisms considered.
2. Description of the Background Art
An internal combustion engine having an intake manifold injector for injecting a fuel into an intake manifold of the engine and an in-cylinder injector for always injecting a fuel into a combustion chamber of the engine, and configured to stop fuel injection from the intake manifold injector when the engine load is lower than a preset load and to cause fuel injection from the intake manifold injector when the engine load is higher than the set load, is known.
In such an internal combustion engine, one configured to switch between stratified charge combustion and homogeneous combustion in accordance with its operation state is known. In the stratified charge combustion, the fuel is injected from the in-cylinder injector during a compression stroke to form a stratified air-fuel mixture locally around a spark plug, for lean combustion of the fuel. In the homogeneous combustion, the fuel is diffused in the combustion chamber to form a homogeneous air-fuel mixture, for combustion of the fuel.
Japanese Patent Laying-Open No. 2001-020837 discloses a fuel injection control apparatus for an engine that switches between stratified charge combustion and homogeneous combustion in accordance with an operation state and that has a main fuel injection valve for injecting a fuel directly into a combustion chamber and a secondary fuel injection valve for injecting a fuel into an intake port of each cylinder. This fuel injection control apparatus for the engine is characterized in that the fuel injection ratio between the main fuel injection valve and the secondary fuel injection valve is set in a variable manner based on an operation state of the engine.
According to this fuel injection control apparatus for the engine, the stratified charge combustion is carried out using only the main fuel injection valve directly injecting the fuel into the combustion chamber, while the homogeneous combustion is carried out using both the main fuel injection valve and the secondary fuel injection valve (or using only the secondary fuel injection valve in some cases). This can keep the capacity of the main fuel injection valve small, even in the case of an engine of high power. Linearity in injection duration/injection quantity characteristic of the main fuel injection valve in a low-load region such as during idling is improved, which in turn improves accuracy in control of the fuel injection quantity. Accordingly, it is possible to maintain favorable stratified charge combustion, and thus to improve stability of the low-load operation such as idling. In the homogeneous combustion, both the main and secondary fuel injection valves are employed, so that the benefit of the direct fuel injection and the benefit of the intake port injection are both enjoyed. Accordingly, favorable homogeneous combustion can also be maintained.
In the fuel injection control apparatus for the engine disclosed in Japanese Patent Laying-Open No. 2001-020837, the stratified charge combustion and the homogeneous combustion are employed according-to the situations, which complicates ignition control, injection control and throttle control, and requires control programs corresponding to the respective combustion manners. Particularly, upon switching between the combustion manners, these controls require considerable changes, making it difficult to realize desirable controls (of fuel efficiency, emission purification performance) at the time of transition. Further, in the stratified combustion region where lean combustion is carried out, the three-way catalyst does not work, in which case a lean NOx catalyst needs to be used, leading to an increased cost.
Based on the foregoing, an engine has also been developed which does not provide stratified charge combustion, and thus does not need control for switching between the stratified charge combustion and the homogeneous combustion and does not require an expensive lean NOx catalyst.
In controlling the engine to be ignited with its coolant having lower temperature, spark advance is introduced for correction. This is because when the coolant has lower temperature (poorer atomization is provided) lower combustion rates are provided and the engine is less prone to knock. The spark advance can provide an increased period of time between ignition and exhaust, and despite lower combustion rates the air fuel mixture can sufficiently be combusted.
In a cold state, however, for a range shared by an intake manifold injector injecting fuel into an intake manifold (or port) having low temperature in a cold state and an in-cylinder injector injecting the fuel into a cylinder having high temperature despite the cold state to both inject the fuel the fuel is atomized differently. As such, the fuel injected through the in-cylinder injector and that through the intake manifold injector have different conditions, respectively. The fuel atomized differently forms air fuel mixtures having different conditions and hence combusting differently, and the air fuel mixtures combusting differently require different optimal timings of ignition. As such, using the coolant's temperature alone to calculate an amount of spark advance cannot provide an accurate timing of ignition (or an accurate amount of spark advance). Furthermore, not only in the cold state but a transitional period from the cold state to a warm state as well, for a range shared by the intake manifold and in-cylinder injectors to both inject the fuel the temperature in the cylinder and that at the intake port increase at different rates. As such, using the coolant's temperature alone to calculate an amount of spark advance cannot provide an accurate amount of spark advance. Note that Japanese Patent Laying-open No. 2001-20837 only discloses that each injector is driven to achieve a fuel injection ratio corresponding to the operation state of interest and a timing of ignition is set, and the document does not provide a solution to the problem described above.